Nowadays there is a growing demand for devices and circuits for processing optical signals for different applications, such as signal processing, image transmission, printing, and the like. Such circuits, which not long ago used discrete devices, namely three-dimensional lenses, glasses, mirrors and the like, are now implemented in an optical integrated circuit criteria, wherein light is confined in a plane in which different optical components are integrated, manufactured on a single substrate. The advantages are reduced size and circuits in which all the optical components are perfectly aligned and are not subject to later undesired misalignment, as might happen with discrete components aligned by mechanical supports.
Such devices are manufactured by fabricating the guides and the other optical components, in which the optical field remains confined, on the surface of a substrate made of a convenient material by localized increase of the refractive index.
One of the methods used for this purpose is ion exchange among the ions of the zones which have to take up guiding properties with the ions of different materials which have the characteristic of rendering the refractive index higher than that of the surrounding substrate. In this case the exchange can take place by dipping the substrate in a melted salt of ions to be incorporated in the optical stratum. Some ions of the chosen zones of the substrate interchange with the ions of the melted salt owing to the different concentration. Therefore these zones develop a different composition and hence a different refractive index from that of the substrate.
The concentration profile of the ions inside the substrate obtained from the exchange, and hence the refractive index profile, is chiefly determined by the melted salt temperature and by the exchange duration time. To obtain profiles different from those due to a mere thermal diffusion phenomenon, an external electric field can be applied to induce a guided ion migration along the force-field lines. That allows a better control of exchange phenomena and hence a better definition of the geometric characteristics of the waveguides and of the other optical components manufactured on the substrate.
According to a method described in the article entitled "One-step field-assisted ion exchange for fabrication of buried multimode optical strip waveguide" by B. G. Pantchev, Electronic Letters, Oct. 22, 1987, pages 1188-1190, the substrate floats on a melted salt, which is the ion source and at the same time the electrode for applying the electric field. The same substrate acts also as bottom wall for an upper box also containing a melted salt, used as the other electrode. In correspondence with the zones in which the ion exchange is required, a thin silver layer is deposited, those ions penetrate into the substrate under the action of the electric field. This method is not easily implemented for two reasons: the difficulty of realizing all the conditions necessary to make the substrate float and the difficulty of laying on the substrate itself the upper box filled with the melted salt, obtaining a stable apparatus, especially taking into account that the upper box is to be connected to the electric supply source.
According to another method, a box for melted salt is fabricated by using as walls the various substrates in which the ion exchange is to be effected. The external surfaces of the substrates are metallized, so as to form one electrode for applying the electric field, and the internal surfaces are masked so as to obtain the desired optical geometry. A conductor dipped in the molten salt is the other electrode for applying the electric field. The method requires the building of this box for molten salt whenever an integrated optical device is to be fabricated, taking all the precautions necessary to build a vessel suited to contain a high temperature liquid and to avoid pollution due to extraneous substances (adhesives and the like), used for fabricating the box or introduced by handling. Hence the method is time consuming and expensive.